Transfer printing processes involve physically transferring an image from one substrate to another, and/or fixing the image on the substrate. Transfer or fixing may involve the application of energy, such as heat energy.
One heat transfer method is melt transfer printing or release transfer printing. A design is first printed on an intermediate substrate using a waxy ink. The back side is then heated with pressure, while the printed side is in close contact with a final substrate. The ink melts onto the final substrate in the mirror image of the original image.
Another method of transfer printing involves inks or toners such as sublimation inks or toners. One form of an appropriate transfer process using liquid sublimation inks is described in Hale, et. al., U.S. Pat. No. 5,601,023, the teachings of which are incorporated herein by reference. An image is generally printed onto a paper media using heat activated dyes. Heat and pressure are applied to the back side of the media, while the image is in close contact with a final substrate. The dyes vaporize, and are preferentially diffused into and/or absorbed by the final substrate to form the image on the substrate. The image transfer of the dye depends on the vapor pressure of the dye and the rate of diffusion of the dye vapor through the layers of the paper, and the affinity as well as physical entrapment of the dye for materials such as binders, fibrous structure, and additives contained in the paper substrate. Due to the nature of the intermediate substrate, which may be a cellulose fibrous material, consumption of the heat activated dyes by the substrate may be substantial. Furthermore, affinity and entrapment of these sublimation colorants may also negatively impact the quality of the image on the final substrate, both in color intensity and in image sharpness. In the case of non-planar substrate transfer, paper substrates also create crumples, wrinkles, and creases that negatively impact image quality, and produce discontinuous images. If the paper substrate is thick, it may not conform to the transfer substrate, negating intimate physical contact, and generating unsatisfactory transfer printing results.
Attempts have been made to achieve sublimation transfer printing on 3-dimensional objects. Intermediate substrates such as thermally shrinkable plastic films and fibrous sheeting materials have been used. Drawbacks of these methods include dimensionally unstable structures, high retention of the heat activate colorants by the transfer substrate due to a high affinity of the substrate material to the colorants, or physical entrapment of color particles, poor heat conductivity, or even physical disruption during the heating process. For example, when using polyester based textile material as the intermediate transfer sheet, sublimation dyes will adhere to textile transfer sheet due to the high affinity between the sublimation dyes and polyester fibrous material. Cellulose based textile intermediate substrates also entrap sublimation dyes during transfer printing process.
Thin metal foils or metallic sheets have also been used as intermediate or image receiving transfer substrates. However, the rigid and continuous nature of metal in the form of a foil or thin sheeting is unsatisfactory in creating and maintaining close surface contact with the final (transferee) substrate, especially when the final substrate comprises curved, spherical, ovoidal or non-flat surfaces. Therefore, such intermediate or transfer substrates are not suitable for many three dimensional applications.